We are interested in the genetic control of cell differentiation. Our research focuses on how T cell receptor (TCR)-aa+CD4+8+ ('double positive', DP) thymocytes differentiate into mature T cells, a sequence of events contemporary with thymocyte 'positive selection', i.e. rescue of DP thymocytes from death by neglect. Positive selection and differentiation both require Major Histocompatibility Complex (MHC)-induced TCR signaling and result in the generation of 'single positive' (SP) T cells that are CD4+8- and helper if class II-MHC specific or CD4-8+ and cytotoxic if class I-MHC specific.In our initial investigations of these events, we demonstrated that the kinetics of intrathymic TCR signaling is an important component of the lineage choice decision in vivo, and specifically that persistent TCR signaling is required for CD4 but not for CD8 lineage choice (Liu and Bosselut, 2004; Liu et al, 2005). In a second approach, we used a microarray screen to identify genes upregulated as DP thymocytes differentiate into SP cells, and therefore of potential importance in that process. We have analyzed in detail the function of one such gene, encoding the zinc finger transcription factor cKrox (also called Zfp67, Thpok or Zbtb7b). We have found that cKrox promotes CD4- at the expense of CD8-lineage differentiation, regardless of MHC specificity (Sun et al., 2005). Analyses of cKrox function in T cell differentiation have been the main focus of the laboratory for the past year. Our current investigations in this area are addressing two specific questions. First, we found that the CD4-differentiating effect of cKrox requires thymocytes to undergo TCR signaling, suggesting that cKrox functionally cooperates with TCR-induced transcription factors to promote CD4 differentiation. Analyses are underway to investigate such functional cooperation. Secondly, we have observed that introducing cKrox in mature CD8 T cells (where it is not normally expressed) affects their phenotypic and functional differentiation. We are currently working to understand the mechanistic basis of this effect.